1. Field of Invention
This invention relates to novel, heat-developable photothermographic elements and in particular, it relates to photothermographic elements containing photographically useful materials with silyl blocking groups.
2. Background to the Art
Silver halide-containing, photothermographic imaging materials (i.e., heat-developable photographic materials) processed with heat, and without liquid development, have been known in the art for many years. These materials, also known as "dry silver" compositions or emulsions, generally comprise a support having coated thereon: (1) a photosensitive material that generates atomic silver when irradiated, (2) a non-photosensitive, reducible silver source, and (3) a reducing agent for the non-photosensitive, reducible silver source, and (4) a binder. The photosensitive material is generally photographic silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that elemental silver (Ag.degree.) is a catalyst for the reduction of silver ions, and a progenitor of the photosensitive photographic silver halide may be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions, such as by partial metathesis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075), coprecipitation of silver halide and reducible silver source material (see, for example, U.S. Pat. No. 3,839,049), and other methods that intimately associate the photosensitive photographic silver halide and the non-photosensitive, reducible silver source.
The non-photosensitive, reducible silver source is a material that contains silver ions. The preferred non-photosensitive reducible silver source comprises silver salts of long chain aliphatic carboxylic acids, typically having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed, and U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image generally is not visible by ordinary means and the photosensitive emulsion must be further processed in order to produce a visible image. The visible image is produced by the reduction of silver ions, which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, i.e. the latent image. This produces a black and white image.
As the visible image is produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable in order to reduce the cost of raw materials used in the emulsion.
A variety of ingredients may be added to these basic components to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic emulsions, as described in U.S. Pat. Nos. 3,846,136; 3,994,732; and 4,021,249.
One conventional way of attempting to increase the maximum image density of photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming materials in the emulsion. Color images can be formed by incorporation of leuco dyes into the emulsion. Leuco dyes are the reduced form of a color-bearing dye. Upon imaging, the leuco dye is oxidized, and the color-bearing dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye enhanced silver image can be produced, as shown, for example, in U.S. Pat. Nos. 3,531,286; 4,187,108; 4,426,441; 4,374,921; and 4,460,681.
Multicolor photothermographic imaging elements typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsion layer. Photothermographic elements having at least 2 or 3 distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4,021,240 and 4,460,681. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are well known in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747; and Research Disclosure, March 1989, item 29963.
One common problem that exists with photothermographic systems is the instability of the image following processing. The photoactive silver halide still present in the developed image may continue to catalyze print-out of metallic silver during room light handling. Thus, there exists a need for stabilization of the unreacted silver halide. The addition of separate postprocessing image stabilizers or stabilizer precursors provides the desired post-processing stability. Most often these are sulfur-containing compounds such as mercaptans, thiones, and thioethers as described in Research Disclosure 17029. U.S. Pat. No. 4,245,033 describes sulfur compounds of the mercapto-type that are development restrainers of photothermographic system. See also U.S. Pat. Nos. 4,837,141 and 4,451,561. Mesoionic 1,2,4-triazolium-3-thiolates as fixing agents and silver halide stabilizers are described in U.S. Pat. No. 4,378,424. Substituted 5-mercapto-1,2,4-triazoles, such as 3-amino-5-benzothio-1,2,4-triazole, used as post-processing stabilizers are described in U.S. Pat. Nos. 4,128,557; 4,137,079; 4,138,265; and Research Disclosure 16977 and 16979.
Some of the problems with these stabilizers include thermal fogging during processing or losses in photographic sensitivity, maximum density, or contrast at effective stabilizer concentrations.
Stabilizer precursors have blocking or modifying groups that are usually cleaved during processing with heat and/or alkali. This provides the primary active stabilizer which can combine with the photoactive silver halide in the unexposed and undeveloped areas of the photographic material. For example, in the presence of a silver halide precursor in which the sulfur atom is blocked upon processing, the resulting silver mercaptide will be more stable than the silver halide to light, atmospheric, and ambient conditions.
Various blocking techniques have been utilized in developing the stabilizer precursors. U.S. Pat. No. 3,615,617 describes acyl blocked photographically useful stabilizers. U.S. Pat. Nos. 3,674,478 and 3,993,661 describe hydroxyarylmethyl blocking groups. Benzylthio releasing groups are described in U.S. Pat. No. 3,698,898. Thiocarbonate blocking groups are described in U.S. Pat. No. 3,791,830, and thioether blocking groups in U.S. Pat. Nos. 4,335,200, 4,416,977, and 4,420,554. Photographically useful stabilizers which are blocked as urea or thiourea derivatives are described in U.S. Pat. No. 4,310,612. Blocked imidomethyl derivatives are described in U.S. Pat. No. 4,350,752, and imide or thioimide derivatives are described in U.S. Pat. No. 4,888,268. Removal of all of these aforementioned blocking groups from the photographically useful stabilizers is accomplished by an increase of pH during alkaline processing conditions of the exposed imaging material.
Other blocking groups which are thermally sensitive have also been utilized. These blocking groups are removed by heating the imaging material during processing. Photographically useful stabilizers blocked with thermally sensitive carbamate derivatives are described in U.S. Pat. Nos. 3,844,797 and 4,144,072. These carbamate derivatives presumably regenerate the photographic stabilizer through loss of an isocyanate. Hydroxymethyl blocked photographic reagents which are unblocked through loss of formaldehyde during heating are described in U.S. Pat. No. 4,510,236. Development inhibitor releasing couplers releasing tetrazoylthio moieties are described in U.S. Pat. No. 3,700,457. Substituted benzylthio releasing groups are described in U.S. Pat. No. 4,678,735; and U.S. Pat. Nos. 4,351,896 and 4,404,390 utilize carboxybenzylthio blocking groups for mesoionic 1,2,4-triazolium-3-thiolates stabilizers. Photographic stabilizers which are blocked by a Michael-type addition to the carbon-carbon double bond of either acrylonitrile or alkyl acrylates are described in U.S. Pat. Nos. 4,009,029 and 4,511,644, respectively. Heating of these blocked derivatives causes unblocking by a retro-Michael reaction.
Various disadvantages attend these different blocking techniques. Highly basic solutions which are necessary to cause deblocking of the alkali sensitive blocked derivatives are corrosive and irritating to the skin. With the photographic stabilizers which are blocked with a heat removable group, it is often found that the liberated reagent of by-product, for example, acrylonitrile, can react with other components of the imaging construction and cause adverse effects. Also, inadequate or premature release of the stabilizing moiety within the desired time during processing may occur.
Thus, there has been a continued need for improved post-processing stabilizers that do not fog or desensitize the photographic materials, and stabilizer precursors that release the stabilizing moiety at the appropriate time and do no have any detrimental effects on the photosensitive material or user of the material.
Silyl groups have long been employed to derivatize and protect various substrates during chemical and synthetic sequences. The silyl protection of a hydroxy group is simply a replacement of the active hydrogen by the silyl group. See, for example, L. Berkofer and A. Ritter, "Newer Methods in Preparative Organic Chemistry," Vol. V, Academic Press, New York, NY, 1968, page 221; A. E. Pierce, "Silylation in Organic Compounds," Pierce Chemical Co., Rockford, Ill., 1968; and J. F. Klebe, Acc. Chem Res., 1979, 3, 299. The technique affords products which are more chemically stable and will undergo subsequent chemical reactions at sites other than the silyl-blocked one.
Simple deblocking of a trialkylsilyl group is well-known in the art. See McOmie, J. F. W. Ed., "Protective Groups in Organic Chemistry," 1975; and Pierce, A. E., "Silylation of Organic Compounds," Pierce Chemical Co., Rockford, Ill., 1968. It is usually effected in aqueous or aqueous methanol media at ambient temperature, reflux, or using acid-catalysis. See C. C. Sweeley, R. Bentley, M. Makita, and W. W. Wells, J. Amer. Chem. Soc. 1963, 85, 2497; A. G. Sharkey, Jr.; R. A. Friedel, and S. H. Langer, Analyt. Chem. 1957, 29, 770. Deblocking under such conditions is usually facile. Such procedures have limitations for those materials which involve siloxane materials in that the latter during storage suffer instability prior to their use.
Fluorinative de-silylation is also known in the art (see, for example, S. J. Brown and J. H. Clark, J. Fluorine Chemistry 1985, 30, 251 and G. G. Yakobson and N. E. Akmentova, Synthesis 1983, 169; M. Gerstenberger and A. Haas Angew. Chem., Int'l Ed. Engl. 1981 20, 647). The procedure generally uses alkali metal salts under ambient conditions or heating depending on the nature of the precursor materials. When used in the de-silylation of siloxylated materials the latter exhibit de-blocking within short period of time. A major factor contributing to the wide acceptance of silyl blocking groups is that both blocking and de-blocking reactions are high-yield reactions and often quantitative.
Although silylation techniques have found application in a wide range of synthetic designs and technologies, silyl blocking groups have heretofore not been effectively employed in protecting the materials of photothermographic and dry-developable imaging. Successful blocking and release of the photothermographically useful materials allows for improved color and black-and-white photothermographic products.